CiteULike: dchen's micelle

Wall slip, shear banding, and instability in the flow of a triblock copolymer micellar solution

Sébastien Manneville — 2008-06-11T16:47:04-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 6. (2007)

The shear flow of a triblock copolymer micellar solution (PEO-PPO-PEO Pluronic P84 in brine) is investigated using simultaneous rheological and velocity profile measurements in the concentric cylinder geometry. We focus on two different temperatures below and above the transition temperature Tc which was previously associated with the apparition of a stress plateau in the flow curve. (i) At T=37.0 °C<Tc, the bulk flow remains homogeneous and Newtonian-like, although significant wall slip is measured at the rotor that can be linked to an inflexion point in the flow curve. (ii) At T=39.4 °C>Tc, the stress plateau is shown to correspond to stationary shear-banded states characterized by two high shear rate bands close to the walls and a very weakly sheared central band, together with large slip velocities at the rotor. In both cases, the high shear branch of the flow curve is characterized by flow instability. Interpretations of wall slip, three-band structure, and instability are proposed in light of recent theoretical models and experiments.

Evidence for three-dimensional unstable flows in shear-banding wormlike micelles

Lydiane Bécu — 2008-06-11T00:06:30-00:00

We report on an experimental study of the shear-banding phenomenon in the concentrated wormlike micellar system CTAB at 20 wt. % in D2O. Time-resolved velocity profiles are recorded using ultrasonic velocimetry simultaneously to global rheological data. Our results confirm the studies performed previously by Fischer and Callaghan [Phys. Rev. E 64, 011501 (2001)]. Time averaged velocity profiles display an unsheared “nematic gel.” In the range of applied shear rate, the flow field exhibits very fast temporal fluctuations. Suspicions for the presence of three-dimensional flow are evidenced and possible causes for a three-dimensional instability are discussed together with the coupling of wall slip to bulk dynamic.

Nonlinear microrheology of wormlike micelle solutions using ferromagnetic nanowire probes

Nathan Cappallo — 2008-06-10T21:28:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 3. (2007)

Ferromagnetic nanowires were employed to investigate the microrheology of wormlike micelle solutions composed of equimolar cetylpyridinium chloride–sodium salicylate. For a wire rotated about a short axis, the drag at low rotation rate R is independent of R and strongly temperature dependent, consistent with the macroscopic shear viscosity. Above a critical rotation rate c, the drag is independent of temperature and decreases as a power law with increasing rate. The onset of nonlinear drag is characterized by a peak associated with contributions from extensional flow. Above c, the fluid generates an additional torque that tilts the wire out of regions of high shear flow and that is interpreted as a consequence of a shear-induced transition to nematic order among the micelles. Rotation of the wire in response to this torque reveals directly the anisotropy of the drag in the nonlinear state.

Spontaneous Formation of Complex Micelles from a Homogeneous Solution

Xuehao He — 2008-05-03T17:52:28-00:00

Physical Review Letters, Vol. 100, No. 13. (2008)

We present an extensive computer simulation study of structure formation in amphiphilic block copolymer solutions after a quench from a homogeneous state. By using a mesoscopic field-based simulation method, we are able to access time scales in the range of a second. A “phase diagram” of final structures is mapped out as a function of the concentration and solvent philicity of the copolymers. A rich spectrum of structures is observed, ranging from spherical and rodlike micelles and vesicles to toroidal and net-cage micelles. The dynamical pathways leading to these structures are analyzed in detail, and possible ways to control the structures are discussed briefly.

Charge Stabilization in Nonpolar Solvents

MF Hsu — 2008-04-23T21:21:25-00:00

Langmuir, Vol. 21, No. 11. (24 May 2005), pp. 4881-4887.

Abstract: While the important role of electrostatic interactions in aqueous colloidal suspensions is widely known and reasonably well-understood, their relevance to nonpolar suspensions remains mysterious. We measure the interaction potentials of colloidal particles in a nonpolar solvent with reverse micelles. We find surprisingly strong electrostatic interactions characterized by surface potentials, e, from 2.0 to 4.4 kBT and screening lengths, -1, from 0.2 to 1.4 m. Interactions depend on the concentration of reverse micelles and the degree of confinement. Furthermore, when the particles are weakly confined, the values of e and extracted from interaction measurements are consistent with bulk measurements of conductivity and electrophoretic mobility. A simple thermodynamic model, relating the structure of the micelles to the equilibrium ionic strength, is in good agreement with both conductivity and interaction measurements. Since dissociated ions are solubilized by reverse micelles, the entropic incentive to charge a particle surface is qualitatively changed from aqueous systems, and surface entropy plays an important role.